This application incorporates by reference of Taiwan application Serial No. 90107637, filed Mar. 30, 2001.
1. Field of the Invention
This invention is related to a hysteresis comparing device, and more particularly, related to a hysteresis comparing device with constant hysteresis width.
2. Description of Related Art
Referring to FIG. 1, it is a block diagram, schematically illustrating a conventional tracking servo control loop. The tracking servo control loop is used in a CD/DVD system for reading information data stored in the CD/DVD disc and outputting the digital signals. The tracking servo control loop 100 includes a pick-up head (PUH) 102, a tracking-error signal generator 104, a low pass filter 106, and a hysteresis comparing device 108. The pick-up head 102 is used to access information signals from the CD/DVD disc, and output two satellite signals Ve and Vf. These two signals have a phase different of 180xc2x0. The tracking error signal generator 104 is coupled to the pick-up head 102 for receiving the satellite signals of Ve and Vf and then generate a tracking signal Va. The low pass filter 102 is coupled to the tracking error signal generator 104 to receive the tracking signal Va, and then generate a low pass signal Vdc in a direct current type. The low pass signal Vdc is an averaged signal level of the tracking signal Va. The hysteresis comparing device 108 has a positive input terminal, which is coupled to the tracking error signal generator 104 to receive the tracking signal Va. A negative input terminal of the hysteresis comparing device 108 is coupled to the low pass filter 106 to receive the low pass signal Vdc. The hysteresis comparing device 108 is used to receive the tracking signal Va and the low pass signal Vdc, and output the digital signal Vd.
Referring to FIG. 2, it is a drawing, schematically illustrating a signal transformation diagram for the hysteresis comparing device 108. In FIG. 2, the horizontal axis represents the tracking signal Va, which is received by the hysteresis comparing device 108 at the positive input terminal. The vertical axis represents digital signal Vd, which is outputed by the hysteresis comparing device 108. When the digital signal Vd is at the low voltage level and the voltage value of the tracking signal Va is gradually increasing up to an upper threshold voltage Vth, the digital signal Vd outputted by the hysteresis comparing device 108 is transformed form a low voltage level to the high voltage level. When the digital signal Vd is at the high voltage level and the voltage value of the tracking signal Va is gradually decreasing down to a lower threshold voltage Vtl, the digital signal Vd outputted by the hysteresis comparing device 108 then is transformed form a high voltage level to the low voltage level. The voltage difference between the upper threshold voltage Vth and the lower threshold voltage Vtl is called as hysteresis width. The hysteresis width usually is, for example, 20 mV. In general, when the voltage difference for a usual comparator from the positive input terminal to the negative input terminal is greater than zero, a voltage with high level is outputted. When the voltage difference for a usual comparator from the positive input terminal to the negative input terminal is less than zero, a voltage with low level is outputted. However, when the voltage difference for the hysteresis comparing device 108 from the positive input terminal to the negative input terminal is at the increasing stage, that is, the voltage difference is gradually increasing, as shown in FIG. 2 with indication of the route A. The output of the hysteresis comparing device 108 is then transformed into the high voltage level only when the voltage at the positive terminal is necessary to be greater than the upper threshold voltage Vth. When the voltage difference for the hysteresis comparing device 108 from the positive input terminal to the negative input terminal is at the decreasing stage, as shown in FIG. 2 with indication of the route D, the output of the hysteresis comparing device 108 is then transformed into the low voltage level only when the voltage at the positive terminal is necessary to be less than the lower threshold voltage Vtl. The hysteresis comparing device 108 is used to prevent glitch at the input terminal from occurring, and prevent a misjudgment on the signal.
Referring to FIG. 3, it is drawing, schematically illustrating a signal waveform for the tracking servo control loop 100. The tracking error signal generator 104 further outputs the tracking signal Va according to the satellite signals Ve and Vf. The tracking signal Va has glitch induced by the interference from the high frequency signal. Due to the hysteresis effect of the hysteresis comparing device, the glitch of the voltage value between the high threshold voltage Vth and the lower voltage threshold Vtl does not affect the outputted result from the hysteresis comparing device, so that the precision for judging the signal can be improved. When the digital signal Vd is at the low voltage level, the tracking signal Va is necessary to be greater than the upper threshold voltage Vth and when the condition is satisfied, the digital signal then is transformed from the low voltage level to a high voltage level. When the digital signal Vd is at the high voltage level, the tracking signal Va is necessary to be less than the lower threshold voltage Vtl and when the condition is satisfied, the digital signal then is transformed from the high voltage level to a low voltage level.
Referring to FIG. 4, it is a circuit diagram, schematically illustrating a circuit structure of the conventional hysteresis comparing device 108. The hysteresis comparing device 108 includes PMOS transistors PC1, PC2, PD1, PD2, NMOS transistors NL, NR, and a current source Ia with a constant direct current. All of the PMOS transistors PC1, PC2, PD1, and PD2 have the source regions that are commonly coupled to the system voltage source VDD. The gate electrode and the drain region of the transistor PD1 are electrically coupled together. The gate electrode and the drain region of the transistor PD2 are electrically coupled together. The gate electrode of the PC1 transistor is coupled to the drain region of the PC2 transistor. The gate electrode of the PC2 transistor is coupled to the drain region of the PC1 transistor. The drain region of the transistor NL is coupled to the drain region of the PC1 transistor. The drain region of the transistor NR is coupled to the drain region of the PC2 transistor, and also coupled to the constant current source Ia in cascade manner. The gate electrode of the transistor NR is the negative input terminal, and the gate electrode of the transistor NL is the positive input terminal. The hysteresis comparing device 108 has two loops. One loop is composed of the transistors PC1 and PC2 to serve as a positive feedback loop. The other one loop is composed of the transistors NL and NR to serve as a negative feedback loop. If the effect of the positive feedback loop is stronger than the effect of the negative feedback loop, then this circuit has the effect of hysteresis. This phenomenon is well known by the skill artisans. However, due to the mobility carried in the transistor, both the threshold voltage and the thickness of the gate oxide layer are easily affected or changed by the fabrication process or the operation temperature. If the transistors PC1 and PD2 are not properly matched, the transistors PC2 and PD1 are not properly matched, or the transistors NL and NR are not properly matched. Then an unstable voltage difference would be caused between the upper threshold voltage Vth and the lower threshold voltage Vtl. As a result, the width of the hysteresis comparing device 108 is not a constant.
It is therefore an objective of the present invention to provide a hysteresis comparing device with constant hysteresis width.
In accordance with the foregoing objective and other objectives of the present invention, the present invention provides a hysteresis comparing device with constant hysteresis width, which can respectively receive a first signal and a second signal and can output a digital signal. The hysteresis comparing device includes a threshold voltage generator, a multiplexer, and a next stage comparator. The threshold voltage generator is used to receive the first signal and output an upper threshold voltage and a lower threshold voltage. The multiplexer coupled to the threshold voltage generator, are used to receive the upper threshold voltage and the lower threshold voltage, and output a multiplexing signal according to the digital signal. The multiplexing signal is the upper threshold voltage or the lower threshold voltage. The next stage comparator has one terminal used to receive the multiplexing signal, and another terminal used to receive the second signal, also and the other one terminal used to output the digital signal.
The threshold voltage generator includes a bandgap reference voltage generator, a transconductance amplifier, a transistor, a reference resistor, a first current mirror, a second current mirror, a first resistor, and a second resistor. The bandgap reference voltage generator is used to produce the bandgap reference voltage. The transconductance amplifier is coupled to the bandgap reference voltage generator, in which the positive input terminal is used to receive the bandgap reference voltage. The gate electrode of the transistor is coupled to an output terminal of the transconductance amplifier, the source region of the transistor is coupled to the negative input terminal of the transconductance amplifier. One terminal of the reference resistor is coupled to the source region of the transistor, and the other terminal is coupled to a ground voltage. The current flowing through the reference resistor is a reference current. The quantity of the reference current is the result of the bandgap reference voltage divided by the reference resistor. The first current mirror has a first terminal coupled to the drain region of the transistor and used to receive the reference current, a second terminal to produce a first mirror current according to the reference current, and a third terminal to produce a second mirror current according to the reference current. A first terminal of the second current mirror, coupled to a second terminal of the first current mirror, are used to receive the first mirror current; and the second terminal of the second current mirror is to produce a second mirror current according to the first mirror current. One terminal of the first resistor is coupled to the third terminal of the first current mirror, and a voltage on this terminal is the upper threshold voltage. The other terminal of the first resistor is to receive the first signal. One terminal of the second resistor is coupled the other terminal of the first resistor. The other terminal of the second resistor is coupled to the second terminal of the second current mirror. The voltage of the other terminal of the second resistor is the lower threshold voltage.